Impact of heat on metabolic and hemodynamic changes in transcranial infrared laser stimulation measured by broadband near-infrared spectroscopy.

Divya D Reddy,F Gonzalez-Lima,Hanli Liu,Sahil S Nalawade,Suvra Pal,Xinlong Wang

doi:10.1117/1.nph.5.1.011004

Abstract

.Transcranial infrared laser stimulation (TILS) has shown effectiveness in improving human cognition and was investigated using broadband near-infrared spectroscopy (bb-NIRS) in our previous study, but the effect of laser heating on the actual bb-NIRS measurements was not investigated. To address this potential confounding factor, 11 human participants were studied. First, we measured time-dependent temperature increases on forehead skin using clinical-grade thermometers following the TILS experimental protocol used in our previous study. Second, a subject-averaged, time-dependent temperature alteration curve was obtained, based on which a heat generator was controlled to induce the same temperature increase at the same forehead location that TILS was delivered on each participant. Third, the same bb-NIRS system was employed to monitor hemodynamic and metabolic changes of forehead tissue near the thermal stimulation site before, during, and after the heat stimulation. The results showed that cytochrome-c-oxidase of forehead tissue was not significantly modified by this heat stimulation. Significant differences in oxyhemoglobin, total hemoglobin, and differential hemoglobin concentrations were observed during the heat stimulation period versus the laser stimulation. The study demonstrated a transient hemodynamic effect of heat-based stimulation distinct to that of TILS. We concluded that the observed effects of TILS on cerebral hemodynamics and metabolism are not induced by heating the skin.

Highlights

The objective of this study was to quantitatively assess transcranial infrared laser stimulation (TILS)-induced thermal effects on metabolic and hemodynamic changes of forehead tissue measured by broadband near-infrared spectroscopy, as well as to confirm/demonstrate the potential role of such thermal effects on hemodynamic changes of forehead tissue determined by bb-NIRS
The current study focused on heat-induced changes of metabolic and hemodynamic signals on the forehead to determine whether thermal effects of TILS would potentially confound PBM effects that we observed previously.[9]
By comparing the chromophore concentration changes caused by both TILS and heat stimulation, we successfully demonstrated the distinction between heat-induced and TILSinduced responses in the hemodynamic and metabolic signals, which enabled us to exclude the potential confounding effect due to laser heating to the subject’s forehead

Summary

Introduction

The concept of using near-infrared or infrared light to modulate biological functions, known as photobiomodulation (PBM), has recently gained rising attention since it may serve as an effective, noninvasive, interventional tool for multiple neural applications in the future.[1,2,3,4] For example, transcranial infrared laser stimulation (TILS) with 1064-nm laser applied to the forehead has served as a particular approach of brain PBM for improving human neurocognitive functions, such as attention, memory, and executive functions.[4,5,6,7,8] A couple of mechanistic studies on TILS were recently reported by Wang et al.,[9,10] supporting the hypothesis that photons at 1064 nm oxidize cytochrome c oxidase (CCO), the terminal enzyme in the mitochondrial respiratory chain. Light absorption of CCO11 effectively contributes to oxygen and energy metabolism in neurons.[12] TILS leads to upregulation of cerebral CCO and hemodynamics as well as increases in cerebral oxygen consumption.[9,10,13] The mechanism of TILS supported/discussed in Refs. 9 and 10 helps us understand the relationship between metabolic and hemodynamic changes,[14] and provides a mechanistic explanation for beneficial neural effects of PBM and/or TILS in a number of medical conditions.[5,6,7,8] TILS leads to upregulation of cerebral CCO and hemodynamics as well as increases in cerebral oxygen consumption.[9,10,13] The mechanism of TILS supported/discussed in Refs. 9 and 10 helps us understand the relationship between metabolic and hemodynamic changes,[14] and provides a mechanistic explanation for beneficial neural effects of PBM and/or TILS in a number of medical conditions.[5,6,7,8]

Objectives

Methods

Results

Discussion

Conclusion